Meiosis
Chapter 13: Meiosis and Sexual Life Cycles
Variation
 Living organisms are distinguished
by their ability to reproduce their
own kind
 Genetics is the scientific study of
heredity and variation
 Heredity is the transmission
of traits from one generation to
the next
 Variation is demonstrated by
the differences in appearance
that offspring show from
parents and siblings
Comparison of Reproductions
 In asexual reproduction, one
parent produces genetically
identical offspring by mitosis
 A clone is a group of genetically
identical individuals from the
same parent
 In sexual reproduction, two
parents give rise to offspring that
have unique combinations of genes
inherited from the two parents
 A life cycle is the generation-togeneration sequence of stages in the
reproductive history of an organism
Genes
 Genes are the units of heredity,
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and are made up of segments of
DNA
Genes are passed to the next
generation through reproductive
cells called gametes (sperm and
eggs)
Each gene has a specific location
called a locus on a certain
chromosome
Most DNA is packaged into
chromosomes
One set of chromosomes is
inherited from each parent
Chromosomes
 Every organism has its own chromosome number
 Somatic (body) cell chromosomes come in pairs
 Called diploid (2n) number of chromosomes
 The two chromosomes in each pair are called
homologous chromosomes, or homologs
 They are the same length and carry genes
controlling the same inherited characters
• 1 from mom
• 1 from dad
 In humans, somatic cells have 46 chromosomes
Chromosomes
 Gametes have only 1 of
each chromosome
 Called haploid (n)
number of
chromosomes
 In humans, sex cells
are haploid: n=23
 A karyotype is an
ordered display of the
pairs of chromosomes
from a cell
Homologous
Chromosomes Exception
 Sex chromosomes are
called X and Y
 Human females have a
homologous pair of X
chromosomes (XX)
 Human males have one X
and one Y chromosome
 The 22 pairs of
chromosomes that do not
determine sex are called
autosomes (found in
homologous pairs)
Fertilization
 Each set of 23 chromosomes
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consists of 22 autosomes and a
single sex chromosome
In an unfertilized egg (ovum),
the sex chromosome is X
In a sperm cell, the sex
chromosome may be either X or
Y
Fertilization is the union of
gametes (the sperm and the egg)
The fertilized egg is called a
zygote and has one set of
chromosomes from each parent
The zygote produces somatic
cells by mitosis and develops into
an adult
Meiosis
 Meiosis is a type of cell
division used to make
gametes (sex cells)
 2 nuclear divisions
• Meiosis I
• Meiosis II
 Begins with 1 diploid
(2n) cell
 Ends with 4 haploid
(n) cells
Meiosis
 Gametes are the only types
of human cells produced by
meiosis
 Meiosis results in one set of
chromosomes in each
gamete (23)
 Gametes are the only
haploid cells in animals
 Gametes fuse (23 + 23) to
form a diploid zygote (46)
that divides by mitosis to
develop into a multicellular
organism
Cell Cycle Review
 G1, S, G2, M
 Mitosis has 4 main
phases
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cells split by
cytokinesis
 Produce 2 identical
cells
 Growth and repair
Meiosis I
 Division in meiosis I occurs in four phases:
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I and cytokinesis
 Meiosis I results in two haploid daughter cells with
replicated chromosomes
 Focus is on splitting homologous chromosomes
Interphase
 Meiosis I is preceded by
interphase, in which
chromosomes are
replicated
 Each replicated
chromosome consists of
two identical sister
chromatids
 Sister chromatids held
together by centromere
 The sister chromatids are
genetically identical
Prophase I
 Chromosomes begin to condense
 Nuclear membrane & nucleoli
dissolve
 Homologous chromosomes pair up
(align gene by gene); this is called
synapsis
 Each pair of chromosomes forms a
tetrad, a group of four chromatids
 Each tetrad usually has one or
more chiasmata, X-shaped
regions where crossing over occurs
 In crossing over, nonsister
chromatids exchange DNA segments
Prophase I: Tetrads
Homologous chromosomes
(each with sister chromatids)
A Tetrad
Metaphase I
 In metaphase I, tetrads
independently line up
across from each other
“sandwiching” the equator
(metaphase plate)
 In mitosis the homologs
made one single line on the
equator
 Microtubules from the
poles are attached to the
kinetochores of each
chromosome of each tetrad
Anaphase I
 Pairs of homologous
chromosomes separate
 One chromosome moves
toward each pole, guided
by the spindle fibers
 Sister chromatids remain
attached at the
centromere and move as
one unit toward the pole
Telophase I
 Reverse of prophase I
 Spindle fibers breaks down
 Chromosomes uncoil
 Nuclear envelope reforms
 In the beginning of telophase
I, each half of the cell has a
haploid set of chromosomes
 Each chromosome still
consists of two sister
chromatids
Cytokinesis & Interkinesis
 Cytokinesis forms two haploid daughter cells
 In animal cells, a cleavage furrow forms
 In plant cells, a cell plate forms
 Each new cell has ½ the genetic information as the
original
 1 chromosome from each pair
 Need a second division for sister chromatids to split
 Each chromosome from the pair is still doubled
 No chromosome replication occurs between the end of
meiosis I and the beginning of meiosis II because the
chromosomes are already replicated
Meiosis I
Meiosis II
 Division in meiosis II also occurs in four phases:
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II and cytokinesis
 Meiosis II is very similar to mitosis
 Focus is on splitting sister chromatids
Prophase II & Metaphase II
Prophase II
 Spindle apparatus forms; nuclear
membrane and nuclelous dissolve
 Chromosomes condense (each still
composed of two chromatids) and
move toward the metaphase plate
Metaphase II
 Sister chromatids are arranged at the
metaphase plate
 The two sister chromatids of each
chromosome are no longer genetically
identical
 The kinetochores of sister chromatids
attach to microtubules extending from
opposite poles
Anaphase II & Telophase II
Anaphase II
 Sister chromatids separate and
move to opposite poles
 Each chromatid is now its own
chromosome
Telophase II
 Nuclei reform
 Spindles break down
 Chromosomes uncoil
 Cytokinesis again separates
the cytoplasm
Final Products
 At the end of meiosis, there are four haploid daughter cells
 Each daughter cell is genetically distinct from the others
and from the parent cell
 Each has 1 chromosome from each homologous pair
 Each will mature into eggs or sperm (gametogenesis)
Meiosis II
Meiosis Distinctions
 Three events are unique to meiosis, and all three
occur in meiosis I:
 Synapsis and crossing over in prophase I
 In metaphase I, paired homologous
chromosomes (tetrads) independently arrange
on either side of the equator
 In anaphase I, homologous chromosomes,
instead of sister chromatids, separate
Meiosis & Variation
 Three mechanisms contribute to genetic variation:
 Independent assortment of chromosomes
• Homologous pairs of chromosomes orient
randomly at metaphase I of meiosis
 Crossing over
• Nonsister chromatids of a tetrad exchange
genetic information
 Random fertilization
• Over 8 million different gametes possible (2^23)
• 70 trillion chromosome combinations possible
for zygotes! (2^23)x(2^23)
Mitosis vs. Meiosis
Mitosis vs. Meiosis
 Mitosis
 1 division
 2 daughter cells
 Exact copies of
parent cells
 Diploid to diploid
 Purpose
• Growth
• Repair
• Asexual
reproduction
 Meiosis
 2 divisions
• 1st separates pairs
 4 daughter cells
 Each unique
 Diploid to haploid
 Purpose
• Make gametes/
sex cells
• Leads to genetic
variation
Review Questions
1. Define genetics and differentiate between heredity and variation.
2. Differentiate between asexual and sexual forms of reproduction in regards to
the life cycles of various organisms.
3. Define the following vocabulary associated with meiosis: gene, gamete, locus,
somatic cell, karyotype, homologous chromosomes, & zygote.
4. Differentiate between autosomes and sex chromosomes.
5. Describe the process of fertilization.
6. Define meiosis and explain why there must be 2 divisions.
7. Define the 4 major phases of meiosis I, along with the important events that
occur during those phases and how they are unique from those phases of
mitosis.
8. Explain the relationship between synapsis, tetrads, chiasmata, and crossing
over.
9. Define the 4 major phases of meiosis II, along with the important events that
occur during those phases and how they are unique from those phases of
mitosis.
10. Describe the 3 events that are unique to meiosis.
11. Name and describe 3 mechanisms that contribute to genetic variation.
12. Describe 5 major differences between mitosis and meiosis as processes.